Device for limiting the exhausting of combustion flue gases at the inlet of a furnace for reheating steel products

ABSTRACT

The invention relates to a device for limiting the combustion fume exhaust towards the outside of a continuous furnace, especially a tunnel furnace, for reheating iron and steel products, especially slabs, at the opening located at the entrance of the furnace. The first row of burners consists of at least two burners ( 2   a,    2   c ) arranged at the same distance (D) from the entrance of the furnace and close thereto, and the burners of the first row of burners are inclined mainly from an angle A towards the discharge area of the furnace.

The present invention relates to a device for limiting the exhausting of combustion flue gases to the outside of a continuous furnace for reheating steel products, particularly steel slabs, especially a tunnel furnace, at the opening located at the inlet of the furnace.

A tunnel furnace generally has its inlet permanently open so as to allow products, coming for example from the continuous casting plant located upstream in the flat product production process, to run into said furnace. The inlet cross section of the furnace is reduced to the minimum so as to limit the exhausting of flue gases and thermal radiation to the outside of the furnace. However, according to the prior art, this does not prevent flue gases from escaping via the opening of the furnace. This deleterious phenomenon is directly due to the overpressure existing in the first heating zone of the furnace, the flue usually located in the homogenizing zone not allowing sufficient draw-off of the flue gases. This results in the walls of the furnace becoming degraded at the charging end and poses problems for the equipment located in the hall near and above the furnace.

One example of the start of a tunnel furnace is shown in FIG. 1 of the appended drawings. The direction in which the products to be reheated run is indicated by the arrows placed at the two ends of that part of the furnace shown. Starting from the end where the products enter, this figure shows three successive heating zones 1 a, 1 b, and 1 c followed by two homogenizing zones 1 d and 1 e. A stack 3 serves to extract the combustion flue gases.

FIG. 2 of the appended drawings shows in greater detail the first heating zone 1 a of the tunnel furnace of FIG. 1. The furnace heating is provided by burners 2 placed on the side walls of the furnace, above and below the products to be reheated. The run direction of the products to be reheated is also indicated by the arrows placed at the two ends of the heating zone. The burners are placed in a staggered fashion—for a burner located beneath the product on one face of the furnace, there is a burner above the product on the opposite face and at the same distance from the inlet of the furnace.

The stack 3 for exhausting the combustion flue gases produced by the burners is in this example placed in the second equalizing zone 1 e.

In FIG. 3 may be seen a curve showing the variation of the pressure in the furnace for the exemplary embodiment shown in FIG. 2. The distance along the furnace from the inlet (zero meter abscissa) to the end of the second equalizing zone (corresponding to the 110 meter abscissa) is plotted on the x-axis. The pressure in the furnace is plotted on the y-axis; the zero value corresponds to the atmospheric pressure outside the furnace, a positive pressure of 5 Pa (pascals) is a pressure in the furnace 5 Pa above the pressure outside the furnace, and a negative pressure of 5 Pa is a pressure in the furnace 5 Pa below the pressure outside the furnace.

It may be seen in FIG. 3 that the relative pressure at the inlet of the furnace is about 8 Pa. This causes the flue gases to be exhausted to the outside of the furnace. In addition, FIG. 3 also shows that the underpressure in the second part of the furnace is relatively large, of the order of −25 Pa, which may result in flue gases coming from downstream zones being sucked back. The pressure difference between the charging end and the second part of the furnace reaches 30 Pa.

Several solutions have already been implemented on tunnel furnaces, without the exhausting of the flue gases being thereby completely eliminated.

One of the solutions employed according to the prior art consists in improving the flue gas draw-off by increasing the dimensions of the flues. Another solution according to the prior art consists of the addition of a second flue gas draw-off at the furnace inlet. FIG. 4 shows an embodiment of this type, the second flue gas draw-off at the furnace inlet bearing the reference 4.

In this embodiment, the pressure in the furnace varies from the furnace inlet according to FIG. 5. This shows that the addition of a second flue gas draw-off at the furnace inlet makes it possible to achieve a more uniform pressure over the length of the furnace, with a pressure difference between the charging end and the rest of the furnace limited to about 15 Pa owing to the fact that the pressure is raised in the second part of the furnace, reaching a value of −10 Pa.

However, the overpressure at the inlet of the tunnel furnace has not been reduced and the risk of flue gas exhausting persists. Moreover, the addition of this second flue gas draw-off at the furnace inlet substantially increases the cost because of this additional equipment.

Another solution according to the prior art consists in bringing the first row of burners closer to the inlet of the furnace. A variant of this configuration, shown in FIG. 6 and in FIG. 7, consists in placing two burners 2 a 1, 2 b 1; 2 c 1, 2 d 1 on each face of the furnace, above and below the product to be reheated, so as to have two pairs of burners facing each other. Two burners 2 a 1 and 2 b 1 are placed on one side of the furnace at the same distance from the furnace inlet—the burner 2 a 1 is placed in the upper part and the burner 2 b 1 is placed in the lower part. Two other burners 2 c 1, 2 d 1 are placed on the opposite side of the furnace and facing each other. The burner 2 c 1 faces the burner 2 a 1, while the burner 2 d 1 faces the burner 2 b 1.

FIG. 7 shows the cross section at the first burners of the furnace illustrated in FIG. 6. The burners 2 a 1 and 2 c 1 are above the products to be reheated and at one and the same height H1, while the burners 2 b 1 and 2 d 1 are placed below the products to be reheated at one and the same height H2.

In this embodiment shown in FIGS. 6 and 7 the pressure in the furnace varies from the furnace inlet according to FIG. 8. It may be seen that this configuration is able to limit the pressure at the furnace inlet to about 2 Pa. In addition, the pressure difference between the charging end and the second part of the furnace remains limited to about 17 Pa, with an underpressure of around −15 Pa in the second part of the furnace.

However, the pressure is negative over the entire length of the furnace, especially just after the inlet, which may draw air into the furnace. This would have the effect of increasing the energy consumption of the furnace and of modifying the oxygen content at the furnace inlet, thereby possibly resulting in increased oxidation of the products to be reheated.

As explained above, the solutions employed according to the prior art are not completely satisfactory.

To provide a solution to this problem, the invention consists mainly of a device for limiting the exhausting of combustion flue gases to the outside of a continuous furnace for reheating steel products, particularly steel slabs, especially a tunnel furnace, at the opening located at the inlet of the furnace, and which is characterized in that the first row of burners is made up of at least one pair of burners placed on each side of the furnace, above the products to be reheated, facing each other at the same distance from the inlet of the furnace and close to the inlet of the furnace, and in that the burners of the first row of burners are angled mainly at an angle A toward the outlet of the furnace relative to the transverse plane perpendicular to the longitudinal axis of the furnace.

Advantageously, the distance from the first row of burners to the inlet of the furnace is between 0.3 and 1.5 m. This distance is preferably less than the internal half-width of the furnace.

The angle A at which the burners are angled relative to a direction orthogonal to the side walls is advantageously between 10 and 45 degrees.

The first row of burners may comprise at least four burners distributed in two pairs of two burners, one placed above the products to be reheated with each burner at one and the same height H1, and the second below the products to be reheated with each burner at one and the same height H2.

The invention consists, apart from the arrangements explained above, of a number of other arrangements that will be explained more fully below with regard to embodiments which are described with reference to the appended drawings but are in no way limiting. In these drawings:

FIG. 1 is an example of a tunnel furnace according to the prior art;

FIG. 2 is an example showing how the burners are fitted in the first heating zone according to the prior art;

FIG. 3 is a graphical representation of the pressure in the furnace along the length of the furnace according to the prior art shown in FIGS. 1 and 2;

FIG. 4 is a second example of a tunnel furnace according to the prior art, which includes an additional flue gas draw-off at the inlet of the furnace;

FIG. 5 is a graphical representation of the pressure in the furnace along the length of the furnace according to the prior art shown in FIG. 4;

FIG. 6 is another example of how the burners are fitted in the first heating zone according to the prior art;

FIG. 7 is a cross-sectional view of the furnace according to the prior art shown in FIG. 6 at the first row of burners;

FIG. 8 is a graphical representation of the pressure in the furnace along the length of the furnace according to the prior art shown in FIGS. 6 and 7;

FIG. 9 is a top view of the inlet of the furnace according to one embodiment of the invention;

FIG. 10 is a graphical representation of the pressure in the furnace along the length of the furnace according to the embodiment of the invention shown in FIG. 9;

FIG. 11 is a front view of the furnace inlet; and

FIG. 12 is a graphical representation of the pressure in the furnace along the length of the furnace according to the embodiment of the invention without the lower burners 2 b and 2 d.

FIGS. 1 to 8 of the drawings relating to the prior art were described above.

FIG. 9 shows a top view of the inlet of a furnace according to one embodiment of the invention. Four first burners 2 a, 2 b, 2 c and 2 d are placed at the same distance D from the furnace inlet, namely two burners on each side of the furnace. The axes of the burners are angled at an angle A toward the outlet of the furnace relative to the transverse plane perpendicular to the longitudinal axis of the furnace. Two burners 2 a and 2 c are located above the products to be reheated and two burners 2 b and 2 d below the products to be reheated.

As shown in FIG. 9, the distance D between the inlet of the furnace and the first row of burners is considered below as being that between the inlet of the furnace, i.e. the outer face of the sheet metal furnace construction perpendicular to the longitudinal axis of the furnace, and the theoretical plane transverse to the furnace passing through the intersection of the axes of the burners with the outer surface of the sheet metal furnace construction.

The particular way of fitting these first burners according to the invention makes it possible to create a curtain formed from the flames and the combustion products of these burners. This curtain acts as a barrier and prevents the ambient air from entering the furnace. The angling of the burners toward the outlet of the furnace according to the invention enables the combustion flue gases to flow toward the furnace outlet and thus prevents the flue gases from being exhausted in the opposite direction, via the furnace inlet. The angling of the burners toward the furnace outlet is limited so as to prevent a draft and intake of ambient air into the furnace.

The following burners 2 remain perpendicular to the longitudinal axis of the furnace.

The opening 6 (FIG. 11) into the furnace, intended for the products 5 to enter the furnace, is greater above the products than below them. Since the products run on rollers, the lower face of the products is always at the same height. However, the upper face of the products is at a variable height, depending on the thickness of the products. Consequently, the opening into the furnace may be reduced below the products so as to prevent any flue gas egress or air ingress, but it must be greater above the products so as to allow the thickest product to pass therethrough.

According to another embodiment of the invention, because of a small opening in the furnace beneath the products to be reheated, the two lower burners 2 b and 2 d placed beneath the products to be reheated are unnecessary. In this configuration, the unit calorific power of the burners 2 a and 2 c may be increased compared with a solution comprising also the two lower burners 2 b and 2 d so as to maintain the required temperature at the furnace inlet.

FIG. 10 is a curve showing the variation of the pressure in the furnace according to the embodiment of the invention shown in FIG. 9 with four burners in the first row of burners. FIG. 12 is a curve showing the variation with the pressure in the furnace according to the other embodiment of the invention, without the burners 2 b and 2 d placed beneath the products to be reheated.

As may be seen in FIG. 10 corresponding to four burners in the first row of burners, and in FIG. 12 corresponding only to the two upper burners in the first row of burners, the invention makes it possible to achieve a pressure close to atmospheric pressure at the furnace inlet, thus preventing the flue gases from being exhausted to the outside of the furnace and a positive pressure just after the furnace inlet, helping to prevent air ingress into the furnace.

The higher pressure just after the furnace inlet shown in FIG. 12 compared with FIG. 10 is due to the fact that, in the case of FIG. 12, the upper burners 2 a and 2 c have a higher power compared with the case shown in FIG. 10 when the lower burners 2 b and 2 d are absent.

The solution adopted according to the invention also makes it possible to limit the pressure difference over the length of the furnace and the underpressure in the second part of the furnace, thus preventing flue gases from being sucked in from the zones located downstream from the second part thereof.

The position of the burners in the first row relative to the furnace inlet and relative to the products to be reheated, the unit power of these burners and the angle at which the burners are angled toward the furnace outlet are defined according to the geometric characteristics of the laboratory (dimensions, position of the flue gas outlet or outlets), according to the combustion characteristics at the burners in this first row and according to those of the burners in the first heating zones (the nature of the fuel, air factor, air/gas flow rates, air and gas temperatures) so as to obtain the expected effect. These parameters are for example obtained from a CFD model of the furnace, solving the equations governing the mechanical behavior of the fluids (conservation of mass, Navier-Stokes equations and turbulence model).

According to one embodiment of the invention, for a furnace with an internal width of 2 m and an internal height of 2.65 m, the burners in the first row are four in number, two burners being fitted facing each other on each side of the furnace above and below the products to be reheated. The burners are fitted 0.7 m from the furnace inlet and have a unit power of 620 kW, the axis of the upper burners being at 575 mm above the transverse axis located at mid-height of the furnace. The axis of the lower burners is at 695 mm below this transverse axis located at mid-height of the furnace and the angle A at which the burners are angled toward the outlet of the furnace is 30 degrees.

According to the invention, the distance D from the angled burners of the first row of burners to the furnace inlet is, depending on the dimensions of the furnace, between 0.3 and 1.5 meters. This distance must however not be too small so as not to cause air to enter the furnace. Nor must it be too large, so as to maintain an effective curtain effect. This distance is for example equal to or smaller than the internal half-width of the furnace.

According to the invention, the angle A at which the burners are angled is between 10 and 45 degrees depending on the dimensions of the furnace.

According to the preferred embodiment of the invention, the burners 2 a, 2 b, 2 c, and 2 d of the first row of burners operate under fixed conditions and at a nominal power. The operating power of these burners is thus kept constant, whereas the operating power of the other burners 2 varies according to the calorific requirement of the furnace.

The angled burners of the first row of burners are according to the invention identical to those located in the first heating zone, or may be of a different power.

The angling of the burners according to the invention is mainly toward the outlet of the furnace. The burners of the first row may be angled toward the top or the bottom of the furnace, for example by a few degrees upward in the case of the burners of the first row located above the products and a few degrees downward in the case of the burners of the first row located beneath the products, but according to the invention such angling is not necessary. 

1. A device for limiting the exhausting of combustion flue gases to the outside of a continuous furnace for reheating steel products, particularly steel slabs, especially a tunnel furnace, at the opening located at the inlet of the furnace, wherein the first row of burners is made up of at least one pair of burners (2 a, 2 c), each burner is placed above the products (5) to be reheated, facing each other at the same distance (D) from the inlet of the furnace and close to the inlet of the furnace, and in that the burners (2 a, 2 c) of the first row of burners are angled mainly at an angle A toward the outlet of the furnace relative to the transverse plane perpendicular to the longitudinal axis of the furnace.
 2. The device as claimed in claim 1, wherein the distance (D) from the first row of burners to the inlet of the furnace is between 0.3 and 1.5 m.
 3. The device as claimed in claim 1, wherein the distance (D) from the first row of burners to the inlet of the furnace is less than the internal half-width of the furnace.
 4. The device as claimed in claim 1, wherein the angle A at which the burners of the first row of burners is angled toward the outlet of the furnace is between 10 and 45 degrees.
 5. The device as claimed in claim 4, wherein the angle A at which the burners of the first row of burners is angled toward the outlet of the furnace is 30 degrees.
 6. The device as claimed in claim 1, wherein the first row of burners comprises at least four burners (2 a, 2 b, 2 c, 2 d) distributed in two pairs of two burners, one (2 a, 2 c) placed above the products to be reheated with each burner at one and the same height (H1), and the second below the products to be reheated with each burner at one and the same height (H2).
 7. The device as claimed in claim 1, wherein the burners (2 a, 2 b, 2 c, 2 d) of the first row of burners operate under fixed conditions and at a nominal power, the operating power of these burners thus being kept constant, whereas the operating power of the other burners (2) varies according to the calorific requirement of the furnace. 